One characteristic of approaching estrus in animals is a rapid rise in plasma estrogens. A predictable physiological effect of these steroids is a marked increase in rate of uterine blood flow. Preliminary observations utilizing a new thermal technique indicate an elevation in blood flow rate in the vagina as well. This vaginal hyperemia is reflected in an elevated thermal conductance of the vaginal tissues. In an attempt to understand more fully the temporal nature of these hormonal and vascular alterations in female, we propose to study the cyclic changes in vaginal thermal conductance and plasma estrogens and luteinizing hormone as determined by radioimmunoassay. The relation of these variables to ovulation will be determined by direct visualization of ovaries in sheep and by rectal palpation of ovaries in dairy cows. In humans, indirect techniques of judging ovulation time will be used.